From German Patent Application No. DE 4338743, a method and a device are known for testing an ultrasonic sensor. For this purpose, a reflecting object is situated on the vehicle at a defined distance from the ultrasonic sensor, the magnitude of the amplitude reflected by this object being evaluated as a measure of the sensitivity of the ultrasonic sensor. From the propagation time of the ultrasonic signal to the stationary obstacle, it is also possible to determine the ambient temperature. A corresponding measurement is possible in those sensors in whose vicinity a corresponding obstacle is actually provided on the vehicle.
In addition, it is known that in the ultrasonic distance measurements the emitted signals are reflected not only by obstacles in the vicinity of the vehicle; a signal may also be reflected by the ground and sent back to the vehicle. In general, this effect is undesirable, because reflections from the roadway surface may be erroneously interpreted as indicating the presence of an obstacle in the vicinity of the vehicle. Thus, it can occur that a warning of an obstacle in the vicinity of a vehicle is outputted even though no obstacle is situated at the supposed location. Concerning this problem, from European Patent No. EP 935765 it is for example known to exclude such faulty detections by deliberately reducing the sensitivity for the possible distance of the ground echo. Correspondingly, the transmit signal can also be adapted in order to avoid false detections due to ground reflections.